A transistor including a semiconductor film which is formed over an insulating surface is an essential semiconductor element for a semiconductor device. Since there is limitation on manufacture of a transistor in terms of allowable temperature limit of a substrate, a transistor including, in an active layer, amorphous silicon that can be deposited at relatively low temperatures, polysilicon that can be obtained by crystallization with use of a laser beam or a catalytic element, or the like is mainly used for a semiconductor display device.
In recent years, a metal oxide showing semiconductor characteristics, which is called an oxide semiconductor, has attracted attention as a novel semiconductor material having high mobility obtained by polysilicon or microcrystalline silicon and having uniform element characteristics obtained by amorphous silicon. The metal oxide is used for various applications; for example, indium oxide is a well-known metal oxide and used as a material of a transparent electrode included in a liquid crystal display device or the like. Examples of such metal oxides showing semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, and zinc oxide, and transistors in each of which a channel formation region is formed using such a metal oxide showing semiconductor characteristics have been known (Patent Documents 1 and 2).